The invention relates to a device for the corona discharge treatment of the surfaces of irregularly shaped thermoplastic moldings, in which the molded parts are affixed to a movable base electrode and moved past a counterelectrode.
Some plastics, such as polyolefins like polyethylene or polypropylene have properties which make good surface adhesion of coating substances very difficult. This is extremely disadvantageous when moldings of complicated shape, such as are used in the automotive industry, are to be processed with surface adhering materials like paints, adhesives and the like. Moreover, permanent combination of the moldings with other materials to form composites is also difficult.
Several methods for the alteration of the surface properties of such plastics are known. These include roughening action, use of chemicals having an oxidizing effect, and a glow or arc discharge as well as corona discharge which act to improve the adhesion properties of the plastic. In general, electrical treatment requires maintenance of a nearly uniform distance between the discharge electrode and the plastic surface. Otherwise, variation of the quality of coating adhesion is produced.
An example of electric discharge processing is described in German Design Patent No. 17 43 687. In this process, an electric discharge from brush electrodes is used to alter the surface properties of plastic webs as the webs travel between the electrodes. Insulators are disposed at the free ends of the electrodes to inhibit or prevent sparkover along the region of electrode sections extending beyond the width of the webs. This arrangement, however, only permits treatment of plastics in web form.
Another device for the electric corona processing of plastic parts of complicated shape is described in DE AS 12 32 333. In this process, plastic tube surfaces are altered by an electrode exposure process which maintains as constant as possible surface-electrode distance. To accomplish this, the tube is positioned on an inner electrode adapted to the shape of the tube interior. The tube and inner electrode are then placed into a form fitting cavity of an outer electrode which has the shape of the outer surface of the tube. The inner surfaces of the outer electrode cavity are disposed at a constant distance from the tube surfaces. Thus, considerable progress in the treatment of parts of complicated shape has been made and high quality control mass production of large numbers of tubes is possible.
The method described in DE AS No. 12 32 333, however, presents difficult problems when it is used to produce irregularly shaped moldings such as those of the automotive industry. In these applications, the molding, e.g., a dashboard, would have to be put on a base electrode fitting the contour of the molding, and a counterelectrode matching its shape and completely enveloping it would have to be provided. Such a procedure is not only costly, impractical and unreasonable, but also will not work for the mass production of differently shaped parts because of the great number of base and counterelectrodes needed and the rapid interchange of the different parts on the production line.
Therefore, it is an object of the invention to develop a device which needs little maintenance, works economically and permits, without complications, corona processing of the surface of plastic parts of very complicated shape for the automotive industry, even in mass production. A further object is that the device be applicable to plastics like polyolefins, e.g., polyethylene and polypropylene.